Ok so Wednesday I will be driving my 02 bonneville across country to its new owner, my sister.

I drove it tonight after having the wheels balanced and rotated mainly because I noticed a vibratio) in the seat meaning coming from back of car.

Well the vibration is still there...at interstate speed it will start off as slight vibration and work up to a very bad shaking then just die out and drive perfectly smooth, then all over again.

What could be causing this, they said everything balanced good and no bent wheels but maybe I should take it back?

Could it be a bad bearing? I would think I would hear noise if that qas the case, which I don't hear anything.

I need to get it fixed by Tuesday so any help would be great.. I plan to get this taken care of yet this weekend if possible.

Thanks

Move the tires around. Could be a shifted belt on a tire.

Already switched fronts and rears when they were balanced. The are directional tires so that'* all I can do. Also even after moving rears to front the vibration still come from back of car. No steering wheel shake what so ever.

So that rules out wheels and tires. How do the brake rotors look? Could be a warped rotor. Any change when you use the brakes? Try two foot driving and holding your speed as you push the brakes. If you feel a little stronger vibration it should be a warped rotor. A bad bearing should make noise, but not always.

Could a warped rotor on the back be enough to cause a vibration even when not using brakes? I will check it out.

Yes, because it will bump the pads. If you get on the brakes it will go away, but if you touch lightly you should feel it more.

Ok thanks I will check that and see what happens. Would be great if thats what it is.

Well brake rotor were warped as they would spin and then grab at a certain spot. Replaced them and it still didn't fix.the problem. Back at wheel balance place having them check the balance again. They said could he wheel bearing but I would really think if they were that bad it would be making noise.

Back wheel bearings are good, front driver is not however...pisses me off as I just replaced that last year, but I doubt that could cause a shake in the back of my seat and the steering wheel be shake free. Back driver brake seems to be dragging so I'm leaning towards that but really hate to have to replace the calipers.

Just got back from driving the car after putting in a new wheel bearing in the front. Of course that did nothing for the vibration, as I figured it wouldnt.

So next question..could a bad motor/transmission mount cause this vibration? And if it did, wouldn't it stop/change if I put it in neutral on the interstate?

The engine seems to have a good amount of movement...if I put it in drive with my foot on the brake and give it some gas the one side of the motor lifts up a good amount..maybe this is normal.

I can not think of anything else that could cause this..all bearings are good, all rotors good, all wheels road force balanced and good, I just don't know where else to look. The vibration is bad enough that if I look over at the headrest of the passenger seat when it starts vibrating, the seat is actually shaking..I can't drive 1800+ miles like that.

Never had this problem a few months ago before I put it up for sale, so of course it has to do it now..